Gallium nitride (GaN) power transistors are increasingly being utilized in high-power amplifier circuits for cellular base stations and other systems to increase efficiency and operating bandwidth. GaN transistors have proven to provide high amplifier performance due to their relatively high power density and relatively high unit current gain frequency, when compared with some of their silicon based counterparts. The higher power density allows for smaller die peripheries for a given level of output power. This may result in lower drain-source capacitance, CDS, and higher output impedances with wider output bandwidth, when compared with silicon devices.
However, GaN transistors also have several disadvantages when compared with silicon based transistors. For example, the current cost of GaN is significantly higher than that of silicon, putting GaN die area and integration at a premium. Further, GaN transistor compression characteristics challenge digital pre-distortion linearization circuits. GaN transistors tend to have a relatively-slow, gradual amplitude compression, and the transmission phase exhibits an expansion during drive up.
Further still, the GaN input characteristics may significantly limit performance. More specifically, GaN input impedance tends to be very low with a high Q-factor, and the gate-source capacitance, CGS, varies considerably over drive. In a Doherty power amplifier with a GaN main amplifier and a GaN peaking amplifier, the GaN peaking amplifier transitions between an off state and an on state depending on the radio frequency (RF) drive level and signal envelope. As these transitions occur, the input impedance exhibits a large variation and may be highly mismatched to a preceding 50 Ohm gain stage. This mismatch may produce considerable reflection and poor Input Return Loss (IRL). Further, the resulting restricted bandwidth on the input can limit the overall amplifier bandwidth. These and other characteristics of GaN transistors make them impractical or unsuitable for use in many conventional amplifier topologies.